The entorhinal cortex and hippocampus have long been implemented in spatial and episodic memory function. The projects described here will advance our knowledge of the neural mechanisms that support computations within this memory circuit and will guide future efforts to ameliorate memory dysfunction. The mechanisms that support computations that manifest as spatial code in the hippocampus are not fully understood. The discovery of grid cells, neurons in the medial entorhinal cortex (MEC) that discharge in a multitude of locations that are positioned at the vertices of a tessellating triangular grid pattern, has led to the theory that spatial coding is generated in the MEC and is transferred to place cells in the hippocampus. Problematic to this theory, recent data has demonstrated that during periods of pharmacological septal inactivation, place cells in the hippocampus retain their firing location even when theta oscillations and grid cell spiking are severely disrupted. These recent studies were conducted in familiar environments where place cell function may be maintained by other associated cortical input. The experiments proposed here examine the respective contribution of grid cell input and theta oscillations to the learning of spatial maps in the hippocampus in a novel environment. We provide preliminary data (figure 2 in research proposal) of hippocampal 'place cell' recordings in a familiar and a novel environment during temporary inactivation of the medial septum with muscimol. We already know that muscimol inactivation of the medial septum simultaneously eliminates theta oscillations and grid cell spatial periodicity (see Figure 1 in dissertation summary). By recording from hippocampal place cells in a novel environment during inactivation of the medial septum we will test whether the generation of spatial maps in the hippocampus requires theta oscillations and/or grid cell inputs. Preliminary data suggest that this is the case, however the non-specificity of muscimol prevent conclusions about relative contribution of grid cell inputs and theta oscillations to the generation of hippocampal spatial maps. In specific AIM1 we propose tests of an optogenetic strategy that uses retrogradely transported EIAV vector to selectively express ARCH in just the projection from the medial septum to the hippocampus or just the projection from the medial septum to the entorhinal cortex. In AIM2, we propose to optogenetically silence the direct medial septum projection to either the hippocampus or the medial entorhinal cortex during place cell and grid cells recordings. This method will allow us to dissociate the contributions of abolishing grid cell inputs to hippocampus and of inactivating direct septal inputs to hippocampus on the generation of the place code in the hippocampus. An alternate optogenetic method to isolate these projections is also described in AIM2. These experiments will significantly advance our understanding of the mechanisms that support spatial computations in the hippocampus and that support spatial and episodic memory function. In addition, the strategy of selectively silencing subcortical projections to limited cortical regionsis a valuable tool that the applicant will use in future research to elucidate functional neural circuitry.